1. Field of the Invention
The present invention relates to a circuit for generating a high voltage pulse having an extremely high voltage and a large content with the aid of semiconductor switches.
2. Related Art Statements
In order to generate plasma, an abruptly raising high voltage pulse of several kV to several tens kV having a very short duration (sometimes 50 nano seconds is required) has to be applied to a load, i.e. a discharge gap provided in a plasma generating reactor.
FIG. 1 is a circuit diagram showing a principal structure of a known high voltage generating circuit. A DC supply source 1 having a high output voltage which is equal to a voltage of an output high voltage pulse is connected across a pulse energy supplying capacitor 3 via a charging resistor 2. The capacitor 3 is connected across a load (discharging site) 5 via a switch 4. After charging the capacitor 3, when the switch 5 is made conductive, energy is transferred from the capacitor 3 to the load 5.
An inductance existing in a path of a current flowing from the capacitor 3 to the load 5 through the conducting switch 4 is denoted by an inductance 6 in FIG. 1. The load 5 is formed by the discharge gap and it generally consisting of a capacitive element. In FIG. 1, for the sake of explanation, this capacitive element of the load 5 is denoted by a capacitor 7 connected in parallel with the discharge gap 5. When the switch 4 is made conductive, a current flows to the capacitor 7 and the capacitor is charged. The larger and steeper this current is, the steeper the output pulse generated across the capacitor 7 becomes. In this manner, a preferable pulse for the plasma discharge can be attained. However, in practice, the switch 4 has a finite switching time and could not be made conductive instantaneously, and the relatively large inductance 6 is always existent in the circuit. Therefore, a raising edge of the output pulse could not be steep and an output pulse having a short duration could not be generated.
In order to solve the above explained problem of the known pulse generating circuit, there has been proposed a magnetic compression circuit utilizing a saturable iron core. FIG. 2 illustrates such a magnetic compression circuit. In FIG. 2, elements similar to those shown FIG. 1 are denoted by same reference numerals used in FIG. 1 and their detailed explanation is dispensed with. A series circuit of saturable iron or magnetic cores 8-1, 8-2 and 8-3 is connected across the switch 4 and the load 5, capacitors 3-1, 3-2 and 3-3 are connected between terminals of these saturable iron cores and a negative terminal of the DC supply source 1, and a saturable iron core 8 is connected across the load 5.
An inductance of the saturable iron core is very high until the core is saturated, and when a product of a voltage and time reaches a predetermined value, the inductance of the saturable iron core decreases abruptly. For the sake of explanation, it is assumed that inductance values of the saturable iron cores 8-1, 8-2, 8-3 and 8 are decreased in this order and the capacitors 3-1, 3-2 and 3-3 have a same capacitance value. After the switch 4 has been made conductive and the saturable iron core 8-1 has been saturated at an instance T0, voltage pulses v1, v2 and v3 appearing across the capacitors 3-2, 3-3 and 7 are successively compressed on a time axis as depicted in FIG. 3. That is to say, the voltage pulse v1 appearing across the capacitor 3-2 begins to increase from the Instance T0 and becomes maximum after a time duration T1. Since the circuit Is designed such that the saturable iron core 8-2 is saturated at such a time instance, the voltage pulse v2 appearing across the capacitor 3-3 begins to raise and becomes maximum after a time period T2. At this time, the saturable iron core 8-3 is saturated and the voltage pulse v3 begins to raise. After a time period T3 which is shorter than the time period T2, the voltage pulse v3 reaches a maximum value. In this manner, the voltage pulse v3 having a sharp raising edge as well as a relatively short pulse width can be applied across the load 5.
As illustrated in FIG. 2, the known high voltage pulse generating circuit including the saturable reactors has a complicated construction. Since a high voltage is applied to all the elements in the circuit, it is required to use special parts, and it is also required to provide a longer insulation distance. Moreover, the DC supply source 1 has to generate a high voltage. In this manner, the known circuit is liable to be large in size and expensive in cost.
In the known high voltage pulse generating circuit, the switch 4 is generally formed by a thyratron which is a kind of vacuum tube. Since the thyratron has a very high switching speed and can be used under a high voltage, the switch 4 can be formed by a single thyratron, and therefore an inductance of the switch 4 is small. However, the thyratron has the following demerits:
(1) The thyratron cannot operate at a high repetition frequency.
(2) The thyratron cannot be self-turned off and thus a limitation is imposed upon designing the circuit.
(3) The thyratron has a short lifetime and maintenance is cumbersome and expensive.
(4) The thyratron requires a heater circuit as well as a gas control, and therefore the overall circuit is liable to be complicated.
(5) The thyratron malfunctions due to jitter and miss-ignition.
Recently semiconductor switches have been developed in accordance with the progress of power electronics, and there have been designed semiconductor switches which can turn-on and turn-off a large current under a high voltage. However, a semiconductor switch has a lower withstand voltage and could not be substituted for the thyratron. A switch is composed of a series circuit of a number of semiconductor switches and a necessary circuit voltage is sheared by these semiconductor switches. In order to turn-on simultaneously the semiconductor switches connected in series, it is necessary to provide special gate driving circuits. Furthermore, a high voltage is applied between the gate driving circuits, and therefore gate power sources and gate control signals have to be isolated from each other. In general, a remarkable advantage could not be attained by only replacing the thyratron by a series circuit of semiconductor switches.
Recently semiconductor switches have been developed in accordance with the progress of power electronics, and there have been designed semiconductor switches which can turn-on and turn-off a large current under a high voltage. However, a semiconductor switch has a lower withstand voltage and could not be substituted for the thyratron. A switch is composed of a series circuit of a number of semiconductor switches and a necessary circuit voltage is shared by these semiconductor switches. In order to turn-on simultaneously the semiconductor switches connected in series, it is necessary to provide special gate driving circuits. Furthermore, a high voltage is applied between the gate driving circuits, and therefore gate power sources and gate control signals have to be isolated from each other. In general, a remarkable advantage could not be attained by only replacing the thyratron by a series circuit of semiconductor switches.
As explained above, in the known high voltage pulse generating circuit, a high DC voltage source is required and all the circuit components are subjected to a high voltage. Moreover, a pulse having a short width could not be produced due to a limitation in switching speed and a circuit inductance, and therefore the magnetic compression circuit has to be used. Then, the circuit becomes large and expensive.
The present invention has for its object to provide a simple and low cost high voltage generating circuit which can generate directly a narrow high voltage pulse raising sharply without using the magnetic compression circuit by effectively utilizing the circuit inductance.
It is another object of the invention to provide a high voltage generating circuit which can generate a narrow and steep high voltage by means of semiconductor switches having turn-off faculty and operating with a relatively low DC voltage source.
According to the invention, a high voltage pulse generating circuit comprises:
a DC voltage source having first and second output terminals;
a first switch having one end connected to said first output terminal of said DC voltage source;
a branch circuit including a free-wheel diode connected across the other end of said first switch and said second output terminal of the DC voltage source; and
a series circuit including an inductance and a second switch and connected in parallel with said branch circuit;
wherein after making said first and second switch on to store inductive energy in said inductance, the energy stored in the inductance is commuted to a load connected across said second switch by turning-off said first and second switches.
In the high voltage generating circuit according to the invention, said first and second switches may be formed by first and second semiconductor switches. In such a case, a low DC voltage is applied to the inductance through the first and second semiconductor switches to store inductive energy in the inductance, and then the first and second semiconductor switches are turned-off to commutate the inductive energy to a load capacitance of a low inductance circuit. By charging the load capacitance abruptly, it is possible to generate a high voltage pulse having a narrow width.
In a preferable embodiment of the high voltage pulse generating circuit according to the invention, said first semiconductor switch is constituted by a semiconductor switching element having a low withstand voltage and said second semiconductor switch is constructed by a series circuit of a plurality of semiconductor switching elements having a high withstand voltage, the number of said plurality of semiconductor switching elements being determined in accordance with an amplitude of an output voltage pulse to be generated. There are further provided a plurality of iron cores, the number of which is equal to that of said plurality of semiconductor switching elements. A primary winding passing through said plurality of iron cores is connected in series with said free-wheel diode, and a plurality of secondary windings each passing through respective iron cores are connected to gates and cathode terminals of respective semiconductor switching elements of said series circuit of semiconductor switching elements. In this case, it is particularly preferable that each of the semiconductor switching elements of said series circuit is formed by a static induction thyristor. However, according to the invention, the semiconductor switching elements may be formed by another semiconductor switching element such as an insulated gate bipolar transistor (IGBT) which has a turn-off faculty.
In a preferable embodiment of the high voltage pulse generating circuit according to the invention, after discharging the energy to the load by turning-off the second switch, the second switch is turned-on again for a very short time period. Furthermore, said first and second switches may be turned off simultaneously or at different timings.